Space-time Defects Research Articles

Towards a quantum field theory description of nonlocal spacetime defects

We propose an ansatz for encoding the physics of nonlocal spacetime defects in the Green’s functions for a scalar field theory defined on a causal set. This allows us to numerically study the effects of nonlocal spacetime defects on the discrete Feynman propagator of the theory defined on the causal set in 1+1 dimensions, and to compare to the defect-free limit. The latter approaches the expected continuum result, on average, when the number of points becomes large. When defects are present, two points with the same invariant spacetime interval can have different propagation amplitudes, depending on whether the propagation is between two ordinary spacetime points, two defects, or a defect and an ordinary point. We show that a coarse-grained description that is only sensitive to the average effect of the defects can be interpreted as a defect-induced mass and wave-function renormalization of the scalar theory.

Defect Wormhole: A Traversable Wormhole Without Exotic Matter

We present a traversable-wormhole solution of the gravitational field equation of General Relativity without need of exotic matter (exotic matter can, for example, have negative energy density and vanishing isotropic pressure). Instead of exotic matter, the solution relies on a 3-dimensional "spacetime defect" characterized by a locally vanishing metric determinant.

Search for quantum gravity using astrophysical neutrino flavour with IceCube

Along their long propagation from production to detection, neutrino states undergo quantum interference which converts their types, or flavours. High-energy astrophysical neutrinos, first observed by the IceCube Neutrino Observatory, are known to propagate unperturbed over a billion light years in vacuum. These neutrinos act as the largest quantum interferometer and are sensitive to the smallest effects in vacuum due to new physics. Quantum gravity (QG) aims to describe gravity in a quantum mechanical framework, unifying matter, forces and space-time. QG effects are expected to appear at the ultra-high-energy scale known as the Planck energy, $E_{P}\equiv 1.22\times 10^{19}$~giga-electronvolts (GeV). Such a high-energy universe would have existed only right after the Big Bang and it is inaccessible by human technologies. On the other hand, it is speculated that the effects of QG may exist in our low-energy vacuum, but are suppressed by the Planck energy as $E_{P}^{-1}$ ($\sim 10^{-19}$~GeV$^{-1}$), $E_{P}^{-2}$ ($\sim 10^{-38}$~GeV$^{-2}$), or its higher powers. The coupling of particles to these effects is too small to measure in kinematic observables, but the phase shift of neutrino waves could cause observable flavour conversions. Here, we report the first result of neutrino interferometry~\cite{Aartsen:2017ibm} using astrophysical neutrino flavours to search for new space-time structure. We did not find any evidence of anomalous flavour conversion in IceCube astrophysical neutrino flavour data. We place the most stringent limits of any known technologies, down to $10^{-42}$~GeV$^{-2}$, on the dimension-six operators that parameterize the space-time defects for preferred astrophysical production scenarios. For the first time, we unambiguously reach the signal region of quantum-gravity-motivated physics.

Interaction of the generalized Duffin–Kemmer–Petiau equation with a non-minimal coupling under the cosmic rainbow gravity

In this study, we survey the generalized Duffin–Kemmer–Petiau oscillator containing a non-minimal coupling interaction in the context of rainbow gravity in the presence of the cosmic topological defects in space-time. In this regard, we intend to investigate relativistic quantum dynamics of a spin-0 particle under the modification of the dispersion relation according to the Katanaev–Volovich geometric approach. Thus, based on the geometric model, we study the aforementioned bosonic system under the modified background by a few rainbow functions. In this way, by using an analytical method, we acquire energy eigenvalues and corresponding wave functions to each scenario. Regardless of rainbow gravity function selection, the energy eigenvalue can present symmetric, anti-symmetric, and symmetry breaking characteristics. Besides, one can see that the deficit angular parameter plays an important role in the solutions.

Nonsingular bouncing cosmology in general relativity: physical analysis of the spacetime defect

In this paper, we describe physical effects occurring in the regularized Robertson–Walker spacetime which can reveal the presence of the defect. Our analysis is based on two main physical quantities: the compressive forces acting on (human) observers and the energy possessed by massive particles and photons during their dynamical evolution. In section , we claim that with a characteristic length scale of the order of the Planck length compressive forces become so intense near the defect that no (human) observer is able to cross it. In section , we show that the energy exhibits an unusual character over a small time interval around the bounce contrasting with the behaviour in the standard cosmology picture. We conclude the paper with some considerations and open problems related to our results.

Magnetic monopole as a spacetime defect

We propose that the only possible realization of a magnetic pole is emergent, it being an artifact of a torsion defect in a curved spacetime. This special phase is built upon a class of degenerate metric spacetime solutions of first-order gravity in vacuum. The (apparent) magnetic charge is shown to have a topological origin, given by a lower-dimensional counterpart of the Nieh-Yan invariant. At the invertible metric phase at a distance, this topological charge gets reflected as the (magnetic) Reissner-Nordstr\"om charge to an asymptotic observer, even though the monopole defect itself remains hidden.

Radially symmetric scalar solitons

A class of noncanonical effective potentials is introduced allowing stable, radially symmetric, solutions to first order Bogomol'nyi equations for a real scalar field in a fixed spacetime background. This class of effective potentials generalizes those found previously by Bazeia, Menezes, and Menezes [Phys.Rev.Lett. 91 (2003) 241601] for radially symmetric defects in a flat spacetime. Use is made of the "on-shell method" introduced by Atmaja and Ramadhan [Phys.Rev.D 90 (2014) 10, 105009] of reducing the second order equation of motion to a first order one, along with a constraint equation. This method and class of potentials admits radially symmetric, stable solutions for four dimensional static, radially symmetric spacetimes. Stability against radial fluctuations is established with a modified version of Derrick's theorem, along with demonstrating that the radial stress vanishes. Several examples of scalar field configurations are given.

M-theory and the Birth of the Universe

In this review article, we first discuss a possible regularization of the big bang curvature singularity of the standard Friedmann cosmology, where the curvature singularity is replaced by a spacetime defect. We then consider the hypothesis that a new physics phase gave rise to this particular spacetime defect. Specifically, we set out on an explorative calculation using the IIB matrix model, which has been proposed as a particular formulation of nonperturbative superstring theory (M-theory).

Non-linear plane gravitational waves as space-time defects

We consider non-linear plane gravitational waves as propagating space-time defects, and construct the Burgers vector of the waves. In the context of classical continuum systems, the Burgers vector is a measure of the deformation of the medium, and at a microscopic (atomic) scale, it is a naturally quantized object. One purpose of the present article is ultimately to probe an alternative way on how to quantize plane gravitational waves.

Non-metric construction of spacetime defects

We describe a spacetime endowed with a non-metricity tensor which effectively serves as a model of a spacetime foam. We explore the consequences of the non-metricity in several f(R) theories.

Nonsingular bouncing cosmology from general relativity: Scalar metric perturbations

We derive the equations of motion for scalar metric perturbations in a particular nonsingular bouncing cosmology, where the big bang singularity is replaced by a spacetime defect with a degenerate metric. The adiabatic perturbation solution is obtained for nonrelativistic hydrodynamic matter. We get the same result by working with conformal coordinates. This last method is also valid for vector and tensor metric perturbations, and selected results are presented. We, finally, discuss several new effects from the linear perturbations of this nonsingular bouncing cosmology, such as across-bounce information transfer and the possible imprint on cosmological perturbations from a new phase responsible for the effective spacetime defect.

Cosmic footballs from superrotations

Superrotations arise from singular vector fields on the celestial sphere in asymptotically flat space, and their finite integrated versions have been argued by Strominger and Zhiboedov to insert cosmic strings into the spacetime. In this work, we argue for an alternative definition of the action of superrotations on Minkowski space that avoids introducing any defects. This involves realizing the finite superrotation not as a diffeomorphism between spaces, but as a mapping of Minkowski space to itself that may be multivalued or non-surjective. This eliminates any defects in the bulk spacetime at the expense of allowing for defects in the boundary celestial sphere metric. We further explore the geometry of the spatial surfaces in the superrotated spaces, and note that they intersect null infinity at the singularity of the superrotation, causing a breakdown in the large r asymptotic expansion there. To determine how these surfaces embed into Minkowski space, a derivation of the finite superrotation transformation is presented in both Bondi and Newman–Unti gauges. The latter is particularly interesting, since the superrotations are shown to preserve the hyperbolic slicing of Minkowski space in Newman–Unti gauge, and this gauge also provides a means for extending the geometry beyond the Bondi coordinate patch. We argue that the new interpretation for the action of superrotations on spacetime motivates consideration of a wider class of celestial sphere metrics and asymptotic symmetry groups.

Exact solutions of vector bosons in the presence of the Aharonov-Bohm and Coulomb potentials in the gravitational field of topological defects in non-commutative space-time

In this paper, we obtain the exact solutions of Duffin-Kemmer-Petiau equation spin-1 in thepresence of the Aharonov-Bohm and Coulomb potential in the gravitational field of cosmic stringand a global monopole, we are study the DKP equation in commutative space-time, We separatedthe variables with the help of Wigner functions and the parity operator, We solved the system ofthe second order differential equation in the case of P = (−1) j+1 , but in the case of P = (−1) j wesolved the system in j = 0. The spectrum of energy in the gravitational field of cosmic string and aglobal monopole are different, and the commutation relation of the angular momentum are obviouslydifferent from Pauli criterion in the Aharonov-Bohm effect, in spin-0 and spin-1 we have the non-commutativity energy by the perturbation theory, the energy is transition from commutative tonon-commutative spac

Nonsingular bouncing cosmology from general relativity

We investigate a particular type of classical nonsingular bouncing cosmology, which results from general relativity if we allow for degenerate metrics. The simplest model has a matter content with a constant equation-of-state parameter and we get the modified Hubble diagrams for both the luminosity distance and the angular diameter distance. Based on these results, we present a Gedankenexperiment to determine the length scale of the spacetime defect which has replaced the big bang singularity. A possibly more realistic model has an equation-of-state parameter which is different before and after the bounce. This last model also provides an upper bound on the defect length scale.

Perturbations to kink-like topological defects in 2D anti de Sitter spacetime

We examine codimension-1 topological defects whose associated worldline is geodesically embedded in AdS2. This discussion extends a previous study of exact analytical solutions to the equations of motion of topological defects in AdSn in a particular limit where the masses of the scalar and gauge field vanish. We study the linear perturbations about the zeroth-order kink-like solution and verify that they are stable. We also discuss general features of the perturbation expansion to all orders.

On a soliton-type spacetime defect

We review the construction of a particular soliton-type solution of the classical Einstein and matter-field equations. This localized finite-energy static classical solution can be interpreted as a single spacetime defect embedded in Minkowski spacetime and may give rise to several new effects. For a Skyrme-type theory with small enough matter-field energy scale compared to the Planck energy scale and for a sufficiently small defect length scale, the existence of a globally regular solution requires a negative active gravitational mass, so that the defect repels a distant test particle (“antigravity”). There also exist “stealth defects” which have a vanishing asymptotic gravitational mass. These stealth defects are, however, not entirely invisible as they bring about a new type of gravitational lensing.

Regularized big bang singularity

Following up on earlier work on the regularization of the singular Schwarzschild solution, we now apply the same procedure to the singular Friedmann solution. Specifically, we are able to remove the divergences of the big bang singularity, at the price of introducing a 3-dimensional spacetime defect with a vanishing determinant of the metric. This particular regularization also suggests the existence of a "pre-big-bang" phase.

A new kind of quantum field theory of (n – 1)-dimensional defects in 2n dimensions

I describe a project to open a new territory of quantum field theory where the fields live not on a space–time manifold but on certain complete metric spaces of (n − 1)-dimensional objects (defects) in a 2n-dimensional space-time M. These metric spaces are ‘quasi Riemann surfaces’; they are formally analogous to Riemann surfaces. Every construction of a 2d conformal field theory (CFT) is to give an analogous construction of a CFT on the quasi Riemann surfaces, and thereby a cft on M. The global symmetry group of the 2d CFT becomes a local gauge symmetry. Ordinary local quantum fields in space–time are constructed by restricting to small objects. The project is based on writing the free n-form in 2n dimensions as the 2d Gaussian model on the quasi Riemann surfaces.

Lensing and imaging by a stealth defect of spacetime

We obtain the geodesics for the simplest possible stealth defect which has a flat spacetime. We, then, discuss the lensing properties of such a defect, and the corresponding image formation. Similar lensing properties can be expected to hold for curved-spacetime stealth defects.

Constraining D-foam via the 21-cm line

We have suggested earlier that D-particles, which are stringy space-time defects predicted in brane-inspired models of the Universe, might constitute a component of dark matter, and that they might contribute to the masses of singlet fermions that could provide another component. Interactions of the quantum-fluctuating D-particles with matter induce vector forces that are mediated by a massless effective U(1) gauge field, the "D-photon", which is distinct from the ordinary photon and has different properties from dark photons. We discuss the form of interactions of D-matter with conventional matter induced by D-photon exchange and calculate their strength, which depends on the density of D-particles. Observations of the hydrogen 21~cm line at redshifts >= 15 can constrain these interactions and the density of D-matter in the early Universe.